The Problem:  House Wrens may destroy eggs and young of other

If you have placed nest boxes for Tree Swallows or bluebirds near shrubs, edges of
woods, or hedgerows, sooner or later you may hear a loud bubbly song coming from a
rather plain-colored brown bird with a stubby tail and long, sharp-pointed bill.  A House
Wren has discovered your boxes.  (If you aren't sure what a House Wren sounds like,
here to view a YouTube video of one in action).

Like Tree Swallows and bluebirds, House Wrens are cavity-nesters that can't make
their own cavities, and though tiny in size House Wrens have developed effective
methods for competing with larger species for these crucial resources, methods that
can mean big trouble for Tree Swallows and bluebirds.  If House Wrens have indeed
discovered your boxes you will need to decide whether to remove the boxes or not.

House Wrens tend to migrate north later than Tree Swallows and bluebirds, often
arriving on the scene when the swallows and bluebirds are already building nests or
have eggs or small nestlings.  If the swallows and bluebirds are lucky the wrens will
find unoccupied cavities in or near the shrubby tangles they prefer, and leave the
other species' nests alone.  However, if the swallow or bluebird nests are themselves
anywhere near shrubby tangles they may be in danger of distruction or takeover by
the wrens.  

Classic signs of House Wren depredation include coarse twigs, typical of wren nests,
placed on top of existing swallow or bluebird nests, eggs with smallish two-holed
punctures found in the nest, or punctured eggs or small dead swallow or bluebird
nestlings on the ground beneath the nest box.

To compound the problem many boxes that House Wrens take over are not used for
nesting at all.  Instead, a male wren may claim several cavities and construct "dummy
nests" of twigs in each one as part of his territorial behavior and to attract females.  A
female will then choose one and add a lining of grasses and feathers before laying a
clutch of white eggs heavily covered with tiny brownish or reddish speckles.

The Best Solution to Prevent Wren Damage: Keep nest boxes meant for Tree
Swallows or bluebirds far away from dense bushes, woods, and hedgerows.

House Wrens are native songbirds, protected by law.  Therefore, adult House
Wrens cannot be killed, or trapped and relocated.  It is also illegal to interfere
with or remove an active wren nest, defined as one with a lining, or eggs, or
However, dummy nests of twigs alone are apparently not considered "active"
in a legal sense, and may be removed.

There is no swallow or bluebird box design that will exclude or deter House
since the tiny wrens can slip inside any box that will admit swallows or bluebirds.

The only sure way to prevent nest destruction or takeovers by House Wrens
is by locating nest boxes intended for Tree Swallows or bluebirds far from
the wrens' preferred habitat, hundreds of feet away if possible.
 This means
you may need to relocate boxes or eliminate some altogether.  We now place our
swallow boxes well out in open fields, and since we adopted this tactic none of our
swallow nests have been destroyed by wrens.  Unfortunately, this option isn't as
acceptable for bluebirds, which prefer at least a scattering of shrubs in the vicinity of
their nests.  

For a very detailed discussion of House Wren deterrent issues and tactics for bluebird
hobbyists click
here for the Sialis web page on the subject.  This page mentions
wren guards, (below) which we haven't tried but which might work for Tree Swallows
when constructed and used as Sialis describes.

Please don't create clusters of closely-spaced boxes.  Some peoples' reaction to
interspecific competition is to put up more boxes, thinking this gives everyone a
chance to nest.  This seldom works.  More is not better when one is trying to prevent
House Wrens (or House Sparrows) from destroying other species' nests.  We suggest
you don't saturate your yard with nest boxes if you are hoping for either bluebirds or
Tree Swallows.  Box clusters can be magnets for both House Wrens and House
Sparrows, and often result in population explosions of these species.

We do need to state that House Wrens are welcomed by many persons, including
ourselves.  We enjoy their summer-long presence at our home.  But we take pains to
use differing habitat preferences to prevent wren competition with other species.  Our
wren box is in a woodsy part of our backyard where no swallows or bluebirds would
attempt to nest.  

The lesson is that boxes for swallows or bluebirds must be placed with care,
and any location found attractive to both wrens and swallows or bluebirds
should probably not be used in the future.

While swallows and bluebirds can co-exist, nesting close together in paired boxes, this
almost never works with House Wrens.  So, if you are managing for Tree Swallows or
bluebirds, make it a rule never to place a box in or near wren habitat.  And if you
weren't aware of the potential problems, and placed boxes where swallow or bluebird
nestings have been damaged or destroyed by House Wrens, consider removing the
boxes permanently from those spots once you legally can.

Learn About Birds at Tree Swallow Nest Box Projects
Reducing House Wren Damage
to Tree Swallow Nestings